Loupes or magnifying glasses such as those of the hand, stand or binocular type are widely used for conducting delicate work.
The hand magnifying glass is inconvenient in that work must be done with one hand with the magnifying glass held by the other hand.
Although the stand magnifying glass is convenient to use in viewing one point at the work site, the glass must be moved when to be used for a somewhat wider work area. Further in the case of the stand magnifying glass, both eyes will be directed toward the object, but it is viewed with one eye, i.e. by the dominant eye. More specifically, an image of the object as viewed by the eye other than the dominant eye is perceived as located at a position quite different from that of the image perceived by the dominant eye, and the two images are fused into an image in the brain for perception. Accordingly, the glass has the problem of causing fatigue to the user when used for a prolonged period of time.
The binocular magnifying glass can be removably attached to the head of the worker and is therefore convenient in that the work can be conducted with both hands. However, the two lenses of the binocular magnifying glass are spaced apart by a fixed distance which is not adjustable, so that the glass has the following problem.
Ophthalmic optically, it is desired that the optical center of each lens be positioned on the visual axis of the corresponding one of the eyes of the user, but the pupillary distance differs from person to person within the range of about 15 mm. If the optical center of the lens is not positioned on the visual axis, a double image is formed by a prismatic action of the lens, with the result that an objectionable fusion image gives a pain to the wearer. Accordingly, the deviation is generally restricted so that the prismatic action of the lens will not exceed 0.5 D (dioptor). In case of binocular magnifying glasses, the allowable value of the deviation is about 1 mm even if the magnification is low, and the allowable value decreases with increasing magnification. The conventional binocular magnifying glass therefore fails to meet the above requirements for many and unspecified persons. Consequently, the deviation of the optical center of the lens from the visual axis, even if slight, causes fatigue to the eye, making the wearer unable to use the binocular magnifying lens for a prolonged period of time and possibly impairing his health.
The lenses of the binocular magnifying glass are housed in a case, which is held to the eyes for use, so that the glass affords only a front view through the lenses. Because the field of view is thus extremely limited, the wearer is unable to recognize his surroundings and is likely encounter a hazard during working.
The magnifying glasses of various types described comprise a convex lens for viewing an enlarged virtual image of an object. To obtain a sharp image, the object must be located at a specified position within the focal distance of the convex lens. For observation, therefore, the glass must be brought close to the object. This entails the problem that a sufficient work space is not available between the object and the lens. Furthermore, the shortened distance between the eye and the object causes fatigue to the eye.
These problems can be solved by using a convex lens having an increased focal distance, but a greatly decreased magnification will then result.
Accordingly, it has been strongly desired to provide a magnifying glass which is free of the above drawbacks for use in the field of precision work.